<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="captcha" method="post">
    <input name="captcha" maxlength="4" minlength="4" required>
    <img src="captcha" />
    <br>
    <button type="submit">提交</button>

</form>
</body>
</html>